Hi,
Thanks for the very detailed review as always.
I noticed that there's now the addition of a stylus section of the review, completely absent from previous reviews of 2-in-1s. I'd really hope that the time taken to cover that section is effectively spent, so I'd like to suggest a few improvements regarding the execution:
1. Please only try to draw diagonal straight lines, slowly, when testing for diagonal jitter. Veering off course due to human error largely renders the results unreliable. A ruler might help with standardization, but by choosing a ruler, every subsequent model should be tested with a ruler.
2. Please test cursive handwriting, because one is generally forced to write larger letters on a screen than on paper due to accuracy issues and writing in block font cannot possibly match the speed of cursive. Anybody serious in handwriting with a digitizer should be using cursive, it's also far more sensitive to aggressive smoothing behavior which can sometimes be more detrimental than jitter. Not at all an issue on iPP but more or less an issue on all the N-trig models that I've tried(worse, most don't have any option to change this behavior, or only have a very crude slider). Almost any digitizer can be accurate if you're extra patient with it, there are drawing apps/macros which add immense stabilization to enable perfect curves at painfully slow speeds, only suitable for certain forms of drawing of course, the key differentiator is whether a digitizer is accurate at any speed, which rules out excessive stabilization as a workaround.
Regarding the standardization of testing handwriting, I personally use the pangram "the quick brown fox jumps over the lazy dog", to be both thorough and efficient, just like font files. I'd write it 5-10 times to get stable results from the screen and stylus combo.
I also try to test with my eyes closed, because the varying lag in inking could mess up your handwriting if you see it happen(you could get used to it but it takes quite a few days in my experience), a longer lag(basically anything longer than iPP's) may mislead you about where your pen tip is during the moment you write. Speaking of which, setting the device to whatever mode provides maximum performance could minimize inking lag.
Of course, writing speed affects the behavior of stabilization, so I'd recommend trying to write at the same speed across devices, but that's hard to gauge or control. A mere thought at this moment.
3. Testing for parallax can also help, my new Spectre x360 14 has better accuracy and lower stabilization overall than my XPS 13 2N1, but strangely the parallax remains at ~1.5-2mm to the left of my tip no matter how I angle the stylus(this is with a certain film applied, which may not be factory default behavior, the film also seems to slightly increase smoothing, which to me is a bad thing, but it does protect the screen and greatly improves writing experience with a paper-like texture). Parallax is a minor issue though and Windows calibration may reduce it enough to make it a non-issue.
Finally, your test seems to be done on the UWP Onenote, so it would be best to do subsequent tests all on the same app. Using MS paint for example will get very different results.